1. Field of the Invention
The present invention relates to a thermal print head which is used in printers, facsimile machines, or the like. More specifically, it relates to a thermal print head comprising two substrates which are installed on a heat radiating metal plate. One substrate is a head substrate of heat resistant insulating material, such as a ceramic, having a heating resistor for printing. The other is a circuit substrate of synthetic resin having a connector to external devices.
2. Description of the Related Art
Heretofore, a type of thermal print head in which a head substrate and a circuit substrate are installed on the surface of a heat radiating metal plate has been widely used. An example such a of the conventional thermal print head is shown in the Official Gazette of Japanese Patent Laid-Open Publication No. Hei 2-286261.
In such an apparatus, the head substrate is made of a heat resistant insulating material, and a heating resistor for printing is formed on the surface of the head substrate. For the purpose of supplying electric current to the heating resistor, a common wiring pattern, an individual wiring pattern, and a plurality of circuit drive elements are also provided on the surface of the head substrate. Further, on the surface of the head substrate, various types of terminal electrodes (hereinafter referred to as head side terminals) are formed for connecting with the circuit substrate.
On the other hand, the circuit substrate, which is made of synthetic resin, has a connector to external devices. On the surface of the circuit substrate, a wiring pattern is provided for connecting the connector with the head side terminals.
FIG. 19 in this specification is a cross-sectional view of such a thermal print head. A head substrate 201 and a circuit substrate 203 are arranged side by side on a heat radiating plate 205. Both of the substrates are arranged in such a manner that an end of the circuit substrate 203 overlaps with head side terminals of the head substrate 201. A presser 207 is screwed to the heat radiating plate 205. The circuit substrate 203 is pressed down by the presser 207 through elastic thread 209. Due to the press, the wiring pattern on the side of the circuit substrate 203 is electrically connected with the head side terminals. More specifically, the presser 207 has a function of making continuity between both of the substrates by pressing the circuit substrate. Further, the presser 207 has a function of protecting both of the substrates.
The structure described above has an advantage of being inexpensive because the area of the head substrate can be made small. The structure also has an advantage of being able to service general purposes. In other words, if the circuit substrate is modified, the structure would be applicable to various types of external devices.
However, conventional thermal print heads had the following problems:
(1) In order to electrically connect the head substrate with the circuit substrate, it is required to have a presser and a plurality of set screws and elastic threads for installing the presser in the aforementioned structure. Therefore, a large number of parts and processing steps are required, whereby it has the disadvantages of being expensive and heavy.
(2) In order to secure the electrical connection of the head substrate and the circuit substrate, the clamping strength of the presser on the heat radiating plate must be enhanced. For this reason, the presser has high rigidity and is screwed at several points in a longitudinal direction.
When the temperature rises under such a condition that the head substrate and the circuit substrate are being pressed by a high degree of clamping strength, the head substrate may warp resulting from the differential thermal expansion of the head substrate and the circuit substrate. Also, due to such a high clamping strength, distortion may arise at the heat radiating plate in a longitudinal direction. Further, as the heat radiating plate and the presser are fixed to each other, distortion may arise due to the differential thermal expansion of the heat radiating plate and the presser. For the reasons described above, nonuniformity in printing may arise which may deteriorate printing quality.
(3) The presser is installed on each upper side of the head substrate and the circuit substrate. In order to prevent the contact of the presser and printing paper, the presser is arranged away from the head substrate's heating resistor for printing. For the reasons described above, the height and width (width in a direction of a right angle to a longitudinal direction) of the thermal print head increase, and the thermal print head resultingly becomes larger and heavier.